fix yaml syntax

Author: MrYamous

service_container/tags.rst

@@ -424,7 +424,7 @@ To answer this, change the service declaration:
 
             MailerSendmailTransport:
                 tags:
-                    - { name: 'app.mail_transport', alias: {'sendmail', 'anotherAlias'}}
+                    - { name: 'app.mail_transport', alias: ['sendmail', 'anotherAlias']}
 
     .. code-block:: xml